The automatic transmission has been a key development in transportation as it eliminates the need for manual shifting to suitable drive gears by the driver. This is especially beneficial in heavy “stop-and-go” traffic which entails frequent shifting of gears to achieve the most efficient forward motion. Rather than frequent shifting by the driver, the automatic transmission performs these changes without the need for driver input.
One problem with automatic transmissions is lower fuel economy relative to manual transmissions. While manual transmissions directly engage the drive gear with the engine through the flywheel, automatic transmissions generate rotation of the transmission drive shaft through a turbine that is rotated by fluid pumped against the turbine vanes. The pumped fluid provides an indirect connection between the engine and the transmission drive shaft. This indirect connection reduces the efficiency of the vehicle drive train with a consequent loss of gas mileage.
A large portion of this “efficiency gap” between manual and automatic transmissions is eliminated by means of a torque converter clutch. At a predetermined parameter, such as a particular engine RPM or transmission fluid pressure, the torque converter clutch engages the engine flywheel. In one embodiment, an electronic solenoid activates to allow the transmission fluid to press against the clutch pressure plate which in turn presses a friction plate against the torque converter housing that is engaged with the flywheel. This action engages the transmission with the engine to substitute the indirect connection with a direct connection and thus increase gas mileage.
While this direct connection increases gas mileage, it also increases the probability of engine vibration and resonance passing from the engine to the transmission and drive train. To prevent vibration transfer, a slip clutch is used. Instead of a complete engagement of the torque converter with the engine, the clutch slips so as to rotate slightly slower (approximately 50 RPM) than the engine flywheel. However, this constant slippage increases the heat of friction which causes wear on the friction pads, pressure plate and the housing of the torque converter and also degrades the transmission fluid.
United States Patent Publication No. 2003/000790 to Ackermann provides grooves on the inner surface of the torque converter housing or alternatively, the pressure plate or friction pad. The grooves are designed to provide access of the friction pad to cooling oil in the torque converter. However, heated oil from the friction pad is mixed with cooler oil from the turbine and is often then returned to the friction pad thus reducing the heat removal capacity of the oil.
Therefore, there is a need for a device that allows for the operation of a slip clutch to reduce transmission of engine vibration while simultaneously improving the heat removal capacity of the oil flowing through the clutch assembly.